1. Field of the Invention
The present invention relates generally to the production of steam from geothermal brine and especially to processes for clarifying flashed, silica-rich geothermal brine before the reinjection thereof into the ground.
2. Discussion of the Prior Art
Large subterranean aquifers of naturally produced (geothermal) steam or hot aqueous liquids, specifically water or brine, are found throughout the world. These aquifers, which often have vast amounts of energy potential, are most commonly found where the earth's near-surface thermal gradient is abnormally high, as evidenced by unusually great volcanic, fumarole or geyser activity. Thus, as an example, geothermal aquifers are fairly common along the rim of the Pacific Ocean, long known for its volcanic activity.
Geothermal steam or water has, in some regions of the world, been used for centuries for therapeutic treatment of physical infirmities and diseases. In other regions, such geothermal fluids have long been used to heat dwellings and in industrial processes. Although efforts to further develop geothermal resources for these site-restrictive uses continue, considerable recent research and development has, additionally, been directed to exploitation of geothermal resources for production of electrical power which can be conducted, often over existing power grids, for long distances from the geothermal sources. In particular, recent steep increases in the cost of petroleum products used for conventional production of electric power, as well as actual or threatened petroleum fuel shortages or embargos have intensified the interest in use of geothermal fluids as an alternative and generally self-renewing source of power plant "fuel."
General processes by which geothermal fluids can be used to generate electric power are known and have been known for some time. As an example, geothermal steam, after removal of particulate matter and polluting gases, such as hydrogen sulfide and ammonia, can be used in the manner of boiler-generated steam to operate steam turbine generators.
Naturally pressurized geothermal brine or water having a temperature of over about 400.degree. F. can be flashed to a reduced pressure to convert some of the brine or water to steam. The steam produced in this manner can then be used to drive steam turbine generators. The flashed geothermal liquid and the steam condensate obtained from power generation are typically reinjected into the ground to replenish the aquifer and prevent ground subsidence. Cooler geothermal brine or water can often be used to advantage in binary systems in which a low-boiling point, secondary liquid is vaporized by the hot geothermal liquid, the vapor produced being used to operate gas turbine generators. The cooled brine is typically reinjected into the ground.
As might be expected, use of geothermal steam is preferred over use of geothermal water or brine for generating electric power because the steam can be used more directly, easily and cheaply. Consequently, where readily and abundantly available, geothermal steam has been used for a number of years to generate commercially important amounts of electric power at favorable costs. For example, by the late 1970's, geothermal steam at The Geysers in Northern California was generating about two percent of all the electricity used in California.
While energy production facilities at important geothermal steam sources, such as at The Geysers, are generally still being expanded, the known number of important geothermal steam aquifers is small compared to that of geothermal brine or water. Current estimates are, in fact, that good geothermal brine or water sources are about five times more prevalent than are good sources of geothermal steam. The potential for generating electric power is, therefore, much greater for geothermal brine and water than it is for geothermal steam. As a result, considerable current geothermal research is understandably directed towards the development of economical geothermal brine and water electric power generating plants, much of this effort being expended towards use of vast geothermal brine resources in the Imperial Valley of southern California.
Although, as above mentioned, general processes are known for using geothermal brine or water for production of electric power, serious problems, especially with the use of highly saline geothermal brine, have often been encountered in practice. These problems have frequently been so great as to prevent the production of electric power at competitive rates and, as a consequence, have greatly impeded the progress of flashed geothermal brine power plant development in many areas.
These severe problems are caused primarily by the typically complex composition of geothermal brines. At natural aquifer temperatures in excess of about 400.degree. F. and pressures in the typical range of 400 to 500 psig, the brine leaches large amounts of salts, minerals and elements from the aquifer formation, the brine presumably being in chemical equilibrium with the formation. Thus, although brine composition may vary from aquifer to aquifer, wellhead brine typically contains very high levels of dissolved silica, as well as substantial levels of dissolved heavy metals such as lead, copper, zinc, iron and cadmium. In addition, many other impurities, particu1ate matter and dissolved gases are present in most geothermal brines.
As natural brine pressure and temperature are substantially reduced in power plant steam production (flashing) stages, chemical equilibrium of the brine is disturbed and saturation levels of impurities in the brine are typically exceeded. This causes impurities and silica to precipitate from the brine, as a tough scale, onto surrounding equipment walls and in reinjection wells, often at a rate of several inches in thickness per month. Assuming, as is common, that the brine is supersaturated with silica at the wellhead, in high temperature portions of the brine handling system, for example, in the high pressure brine flashing vessels, heavy metal sulfide and silicate scaling typically predominates. In lower temperature portions of the system, for example, in atmospheric flashing vessels, amorphous silica and hydrated ferric oxide scaling has been found to predominate. Scale, so formed, typically comprises iron-rich silicates, and is usually very difficult, costly and time consuming to remove from equipment. Because of the fast growing scale rates, extensive facility down time for descaling operations may, unless scale reducing processes are used, be required. Associated injection wells may also require frequent and extensive rework and new injection wells may, from time to time, have to be drilled at great cost.
Therefore, considerable effort has been, and is being, directed towards developing effective processes for eliminating, or at least very substantially reducing, silica scaling in flashed geothermal brine handling systems. One such scale reduction process, disclosed in U.S. Pat. No. 4,370,858 to Awerbuck, et al, involves the induced precipitation of scale-forming materials, notably silica, from the brine in the flashing stage by contacting the flashed brine with silica or silica-rich seed crystals. When the amount of silica which can remain in the brine is exceeded by the brine being flashed to a reduced pressure, silica leaving solution in the brine deposits onto the seed crystals. Not only do the vast number of micron-sized seed crystals introduced into the flashing stage provide a very much larger surface area than the exposed surfaces of the flashing vessels but also the silica from the brine tends to preferentially deposit onto the seed crystals for chemical reasons. Substantially all of the silica leaving the brine therefore precipitates onto the seed crystals instead of precipitating as scale onto vessel and equipment walls and in injection wells.
Preferably, the seed crystals are introduced into the high pressure flashing vessel, or crystallizer, wherein high pressure, two phase brine is separated. The silica removal or crystallization process, although commencing in the high pressure flash crystallizer, continues in successive, lower pressure flashing vessels in which additional two phase brine separation occurs. In a downstream reactor-clarifier, the silicious precipitate is separated from the brine as a slurry which may contain about 30 percent by weight of silica. According to known processes, a portion of this silicious slurry from the reactor-clarifier stage is recirculated back upstream into the high pressure flash crystallizer, wherein the silica material in the slurry acts as seed material.
For such reasons as aquifer replenishment and avoiding ground subsidence, the brine overflow from the reactor-clarifier, as well as steam condensate from the electric power generating facility, is usually pumped back into the ground through deep injection wells. Typically, however, the clarified brine from the reactor-clarifier still contains too high a concentration of residual suspended solids to be reinjected without causing problems in the injection wells. Although the reactor-clarifier is ordinarily efficient in removing the bulk of the silicious solids contained in the brine. when discharged from the flash crystallization stage, the brine still contains many suspended particles which are too fine to settle out in the reactor-clarifier. The concentration of these fine particles in the brine overflow from the reactor-clarifier is frequently sufficient to cause plugging of the injection wells at an excessive rate. Therefore, absent further treatment of the clarified brine, costly injection well rework and/or the costly drilling of new injection wells may be so frequently required that electric power production by the brine becomes uneconomical.
Consequently, to protect injection wells, a clarified brine filtration stage is usually provided between the reactor-clarifier and the injection wells. When properly functioning, the brine filtration stage, which typically comprises one or more media filters, reduces the residual suspended solids concentration in the brine to acceptable injection levels, it being appreciated that tradeoffs generally exist between the cost of increasing filter effectiveness and the cost of occasional injection well rework.
By way of illustrative example, it has been found that the clarified brine overflow from the reactor-clarifier may in some instances have a residual suspended solids concentration of about 150 parts per million, with a mean particle size of between about 4 and about 5 microns. By effective filtering of the clarified brine, the residual suspended solids concentration may be reduced to about 10 or 15 parts per million, with a mean particle size of between about 3 and about 4 microns. Such solids concentrations after brine filtering appear not to cause an excessive amount of damage to brine injection wells and are generally considered acceptable.
Although it is generally possible, by filtering, to attain such a reduced residual suspended solids content in previously clarified brine, the filtering process has itself typically been found to create new and serious problems in the brine handling system. For filters to be effective in filtering the brine, they must, of course, remove substantial amounts of the residual solids concentration from the brine. These removed materials accumulate in the filters and must periodically be removed for filter efficiency to be maintained. However, it has been the general experience that the fine silicious particles removed by the filters from the clarified brine are very sticky or cohesive in nature and tend rapidly to agglomerate in the filters into sizeable clumps of material commonly referred to as "mud balls." These mud balls, being larger and more massive than the filter media particles, cannot be easily removed from the filters by conventional backwash procedures. It has, in fact, been found that even with frequent backwashing, the mud balls still form at rates requiring the replacement of the filter media every few months, at considerable cost in terms of media replacement, labor and equipment downtime. Available filters have, moreover, been found to be difficult to repack with filter media.
Frequent filter backwashing, to retard the formation of mud balls in the filters and prolong filter media life, however, creates other problems. For example, to avoid the necessity of power plant shutdown during filter backwashing, which may be required every few hours, otherwise redundant filters must generally be provided at substantial added cost. Moreover, frequent filter backwashing creates problems relating to backwash disposal. Ordinarily, the filters are backwashed, from a backwash holding tank, with filtered brine from the filters. Like the brine itself, the backwash brine must generally be reinjected as the only practical method of disposal, particularly since the material backwashed from the filters may contain excessive amounts of such heavy metals as lead and zinc, thereby making other means of backwash disposal impractical.
However, because of the amount of solids suspended in the backwash brine, direct injection thereof through injection wells is also not practical. It has, therefore, been the usual practice to pond the backwash brine for a period of time during which some of the solids settle from the brine, and them to pump the brine, still containing suspended solids, back upstream, for example, into the atmospheric flash vessel for recombination with the main flow of brine upstream of the reactor-clarifier stage. However, the additional fine solids suspended in the backwash brine tends to upset the brine-solids separation process in the reactor-clarifier, thereby causing the clarified brine overflow from the reactor-clarifier to have higher than normal concentrations of suspended solids, in turn, overloading the filters and accelerating the formation of mud balls in the filters. Furthermore, the ponding of the backwash brine before the combining thereof with the main flow of brine causes the brine backwash to become more acidic, due principally to the air oxidation of ferrous ions naturally present in the brine to ferric ions. As a result, brine handling equipment corrosion is typically increased.
As a result of these and other filtering stage problems, the economical production of electric power by use of silica-rich geothermal brine may still be jeopardized and improved processes for the pre-injection treatment of clarified brine are clearly needed.
It is, therefore, an object of the present invention to provide a process for the secondary clarification of geothermal brine prior to the reinjection thereof into the ground.
Another object of the present invention is to provide a geothermal brine secondary clarification process utilizing flocculants and the recirculation of brine underflow from the secondary clarification process.
Other objects, advantages and features of the present invention will become apparent to those skilled in the art from the following description, when taken in conjunction with the accompanying drawings.